1. Technical Field
The present invention relates to a power generation system using molten carbonate fuel cells.
2. Background Art
A fuel cell directly transforms chemical energy of fuel into electrical energy. There are proposed a lot of power generation systems using fuel cells and a molten carbonate fuel cell system is a typical one of them. The molten carbonate fuel cell includes a stack of fuel cells with separators being interposed between two adjacent fuel cells. The fuel cell generally includes an anode (fuel electrode), a cathode (oxygen electrode) and an carbonate-soaked electrolyte (tile) sandwiched by the anode and the cathode. Oxidant gas is supplied to the cathode and fuel gas is supplied to the anode such that the power generation takes place due to a potential difference between the cathode and the anode.
One example of the conventional molten carbonate fuel cell power generation system is illustrated in FIG. 9 of the accompanying drawings. This system is generally called "cathode recycle type". The fuel cell FC includes a plurality of fuel cells. Each fuel cell includes an electrolyte tile 1 sandwiched by a cathode 2 and an anode 3. The fuel cells are stacked with separators being interposed between two adjacent fuel cells. Air A is pressurized by a blower 5 provided on an air feed line 4 and preheated by an air preheater 6 before it is fed to the cathode 2 as the oxidant gas. Cathode exhaust gas discharged from the cathode 2 flows through a line 7 to the air preheater 6 and a heat exchanger 8 and then is expelled to atmosphere. Part of the cathode exhaust gas is fed back to the air feed line 4 via a branch line 9 and recirculated to the cathode 2 again. Another part of the cathode exhaust gas is introduced to a combustor section 12 of a reformer 11 by a line 10 branched from the line 7.
On the other hand, natural gas NG supplied to the anode 3 of the fuel cell FC is mixed with part of reformed gas, which is discharged from a reforming section 13 of the reformer 11 into a reformed gas line 14, and accordingly sulfur or sulfides contained in natural gas is transformed to hydrogen sulfide. The hydrogen sulfide, which has passed through a natural gas preheater (furnace) 15, is desulfurized by a desulfurizer 16, and the natural gas, after the desulfurization, is introduced to an ejector 19. The ejector 19 guides thereinto steam from the boiler 17 through a steam line 18. The steam is introduced to the reforming section 13 with the natural gas. In the reforming section 13, the natural gas and the steam undergo a reforming reaction to be transformed to hydrogen, carbon monoxide, carbon dioxide and so on. Most of reformed gases is led to the anode 3 of the fuel cell FC through another reformed gas feed line 20 branched from the reformed gas line 14. Anode exhaust gas discharged from the anode 3 contains unreacted H.sub.2 and CO so that the anode exhaust gas is introduced to the combustion section 12 of the reformer 11 through an anode exhaust gas line 21 and combusted therein with air contained in the cathode exhaust gas of the branch line 10. Combustion exhaust gas discharged from the combustion section 12 of the reformer 11 enters the boiler 17 and then a condenser 23 through an exhaust gas line 22. The exhaust gas is cooled and condensed to water by the condenser 23. Then, the gas and the water are separated from each other by a gas-liquid separator 24. The gas is pressurized by the blower 25 and fed to the cathode 2 through the air feed line 4. In this manner, a large amount of oxidant gas is recirculated to the cathode 2 to cool the fuel cell stack.
In the above-described fuel cell system, the oxidant gas containing CO.sub.2 and O.sub.2 is fed to the cathode 2 so that a reaction of: EQU CO.sub.2 +1/2O.sub.2 +2e.sup.- .fwdarw.CO.sub.3.sup.--
takes place at the cathode 2, thereby producing carbonate ion CO.sub.3.sup.--. CO.sub.3.sup.-- transports in the electrolyte plate 1 and reaches the anode 3. On the other hand, the reformed gas containing H.sub.2, CO and CO.sub.2 supplied to the anode 3 undergoes a reaction of: EQU CO.sub.3.sup.-- +H.sub.2 .fwdarw.H.sub.2 O+CO.sub.2 +2e.sup.-
whereby hydrogen gas is mainly consumed to produce water and carbon dioxide.
The anode exhaust gas contains unreacted H.sub.2 and CO. Thus, the anode exhaust gas is introduced to the combustion section 12 of the reformer 11 and mixed with the cathode exhaust gas containing the unreacted O.sub.2 to be burned. Heat generated upon this combustion causes the reforming reaction at the reforming section 13 whereby the natural gas NG is reformed to H.sub.2, CO and CO.sub.2.
In the above-explained cathode recycle type power generation system, the pressure difference between the anode 3 and the cathode 2 becomes small since the anode exhaust gas and the cathode exhaust gas are mixed to each other in the combustion section 12 of the reformer 11, and a high power generation efficiency is obtained by raising a concentration of the carbon dioxide of the cathode 2 entrance since the combustion exhaust gas from the combustion section 12 of the reformer 11 and the cathode exhaust gas are recirculated to the cathode 2.
FIG. 10 also a conventional power generation system. This one is generally called "anode recycle type". An anode recycle loop is formed by the anode 3 of the fuel cell FC, the anode exit gas line 21, the blower 26, the reforming section 13 of the reformer 11 and the reformed gas line 14 which feeds the reformed gas to the anode 3. The natural gas NG flows through the desulfurizer 16 and the heat exchanger 15 before reaching the anode exit gas line 21 of the anode recycle loop. On the other hand, the oxidant gas to be fed to the cathode 2 is, after pressurized by the blower 5, mixed with the combustion exhaust gas discharged into the exhaust gas line 22 from the combustion section 12 of the reformer 11 and then fed to the cathode 2 through the air feed line 4. Part of the cathode exit gas is guided to the boiler 17, in which it is subjected to an heat exchange with the water fed from a pump 27 thereby producing steam which is used for certain purposes before expelled to the atmosphere. Part of the cathode exit gas is introduced to the combustion section 12 of the reformer 11 through the cathode exit line 7 such that the unreacted gases of the anode gas and the cathode exit gas are combusted at the combustion section 12 to produce heat, which heat is absorbed by the reforming section 13 to ensure the reforming reaction.
Conventionally, an internal-reforming-type fuel cell has been also developed. This fuel cell functionally combines the fuel cell stack with the reformer. As illustrated in FIGS. 11 and 12, the anode 3 having a rib 28 is employed. The fuel cell elements (each element includes the cathode 2, the anode 3 and the electrolyte plate 1 sandwiched by the cathode 2 and the anode 3) are stacked via the separators 31, like other conventional fuel cells, but since the each anode 3 has the rib 28, fuel gas passages 29 are formed by the ribs 28. Reforming catalyst (alumina substrate and Ni) 30 is placed in the passage 29 so that the passage 29 serves as the reformer. The natural gas and the steam are fed into the passage 29 as the fuel gas and the oxidant gas is fed to a gas passage 32a defined by the separators 31 whereby the reforming reaction at the anode 3 and the electrochemical reaction between the anode 3 and the cathode 2 takes place simultaneously. Meanwhile, numeral 32b designates a cooling air passage.
In the above-described internal-reforming-type fuel cell, the reforming reaction and the electrochemical reaction occur at the same time. Therefore, the reforming reaction proceeds beyond a methane conversion ratio determined by chemical equilibrium temperature since hydrogen and carbon monoxide are consumed at these reactions. Accordingly, a high methane conversion ratio is obtained at a relatively low temperature. In addition, the electrochemical reaction is an exothermic reaction while the reforming reaction is an endothermic reaction so that a high power generation efficiency is achieved by a balanced operation.
However, in the cathode recycle type system shown in FIG. 9, the generating end efficiency is 45.5% at HHV (anode fuel utilization efficiency is 80%, current density is 150 mA/cm.sup.2 and reformer steam/carbon ratio (s/c ratio) is 3) and the cell output voltage is 0.69 V. These values are considered to be low values. This is because although the combustion exhaust gas discharged from the combustion section 12 of the reformer is condensed to remove the water and is recirculated to the cathode 2 with part of the cathode exit gas, which part of the cathode exit gas is cooled before it is recirculated to the cathode 2, in order to raise the CO.sub.2 concentration at the cathode entrance, the CO.sub.2 concentration at the cathode entrance reaches only 11.3%, i.e., the CO.sub.2 concentration is still low. Also, when a flow rate of the air pressurized by the blower 5 and the air pressurized by the other blower 25 and then recirculated to the cathode 2 is set to 20 mol/m, the flow rate of the gas at the cathode exit becomes as much as 17.6 mol/m so that the power consumption of the blowers 5 and 25 is raised and as a result the power consumption of the system is raised and the sending end efficiency is lowered. In addition, since the cell output voltage is low, it is not possible to raise the current density and therefore the plant cannot be designed to be compact.
In the anode recycle type system of FIG. 10, the cooling of the fuel cell stack mainly depends on the anode recycle gas and the heat required for the reforming reaction mainly depends on the sensible heat (of a temperature of about 700.degree. C.) of the anode exit gas. Therefore, there are following disadvantages:
(i) The anode exit gas which contains reacted gas such as KOH or carbonate vapor poisons the catalyst of the reforming section of the reformer 11;
(ii) The anode recycle loop is a high temperature recycle loop and the loss of the blower 26 used for the anode recycle loop increases as the gas temperature rises if the pressure loss of the anode recycle loop is constant. Specifically, there is a loss of three times ((273+550)/273.apprxeq.3) as much as the one of ordinary or room temperature;
(iii) Since the anode gas which contains low concentration of H.sub.2 and CO is recirculated, the H.sub.2 and the CO concentrations at the anode entrance are lowered which in turn lowers the cell voltage; and
(iv) At the anode 3, the carbonate ion CO.sub.3.sup.-- from the cathode 2 reacts with the hydrogen gas to produce H.sub.2 and CO.sub.2 so that the temperature boundary layer becomes thicker. Thus, it is disadvantageous to perform the cooling of the fuel cel stack on the anode 3 side since the heat transfer coefficient of the anode 3 is small as compared with the cathode 2.
Furthermore, in the internal reforming type fuel cell system of FIG. 11, since the reforming catalyst 30 is placed in the gas passage of the anode 3, the catalyst becomes wet due to the carbonate. Accordingly, the catalyst is damaged. Also, the control of the temperature inside the fuel cell stack is difficult.